The FTC is also of a well known type, comprising a tower for cooling fumes by spraying water into said fumes at the outlet of said fume main, and at least one reactor for the physico-chemical neutralization of the fumes by putting the fumes in contact with a powder reagent, such as alumina, then filtering the loaded reagent, in particular by adsorption of fluorinated compounds following contact with the fumes and filtration of the fume dust, and recycling in said reactor of at least one fraction of the filtered reagent and mixing of this with fresh reagent.
It is known that the anodes and cathodes used for aluminium electrolysis are carbon blocks necessary to the chemical reaction of electrolysis of alumina (Al203) in order to obtain aluminium (Al). The base materials used in the production of such anodes or cathodes are calcined petroleum coke constituting the aggregates, and coal pitch, used as a liquid binder. Green blocks are first produced by intensive mixing of the base materials and compaction in order to shape the paste formed from these two main constituents and then baked for approximately one hundred hours at a temperature of the order of 1100° C. The effect of this heat treatment is to transform the coal pitch into coke and consequently to confer on the anodes and cathodes satisfactory mechanical resistance and electrical conductivity for their use in an electrolysis cell.
These green carbonaceous blocks are baked in baking furnaces of a well-known type, called a “ring pit furnace” or “ring furnace” which are used so as to implement the principle of a counter-current gas-solid heat exchanger.
Baking furnaces (BF) for anodes are described in particular in the following patent documents: U.S. Pat. No. 4,859,175, WO 91/19147, U.S. Pat. No. 6,339,729, U.S. Pat. No. 6,436,335 and CA 2 550 880, to which reference will be made for further details in this regard. A reminder of their structure and operation will nevertheless be given, with reference to FIGS. 1 and 2 hereafter, representing respectively a diagrammatic plan view of an open top ring furnace, having two fires in this example, for FIG. 1, and a partial perspective view with cutaway, showing the internal structure of such a furnace, for FIG. 2.
The baking furnace (BF) 1 comprises two parallel shells or sections 1a and 1b, extending along the longitudinal axis XX over the length of the furnace and each comprising a succession of transverse chambers 2 (perpendicular to the axis XX), separated from each other by transverse walls 3. Lengthwise, i.e. in the transverse direction of the furnace 1, each chamber 2 is constituted by alternately juxtaposed pits 4, open at their upper part in order to allow for the loading of the carbonaceous blocks to be baked and the unloading of the cooled baked blocks, and in which the carbonaceous blocks 5 are stacked for baking, packed in a carbonaceous powder, and thin heating flue walls 6. The flue walls 6 of a chamber 2 run on longitudinally (parallel to the major axis XX of the furnace 1) from the flue walls 6 of the other chambers 2 of the same section 1a or 1b and the flue walls 6 communicate with each other by apertures 7 in the upper part of their longitudinal walls, opposite longitudinal passages arranged at this level in the transverse walls 3, such that the flue walls 6 form rows of longitudinal compartments, arranged parallel to the major axis XX of the furnace and in which gaseous fluids (combustion air, combustible gases and combustion gases and fumes) will flow, making it possible to ensure the pre-heating and baking of the anodes 5. The flue walls 6 comprise moreover a device 8 for extending and more uniformly distributing the path of the combustion gases or fumes, and these flue walls 6 are provided, in their upper part, with openings 9, called “ports”, capable of being closed by removable covers.
The two sections 1a and 1b of the furnace 1 communicate at their longitudinal ends by crossovers 10, which make it possible to transfer the gaseous fluids from one section to the other, and sometimes even from the end of a group of rows of flue walls 6 of one section 1a or 1b to the corresponding end of the group of rows of flue walls 6 of the other section 1b or 1a. 
The operating principle of ring furnaces, also called “fire advance furnaces” consists of causing a flame front to move from one chamber 2 to another that is adjacent thereto during one cycle, each chamber 2 successively undergoing phases of preheating, forced heating, full firing, then cooling (natural then forced).
Baking of the anodes 5 is carried out by one or more fires or fire groups, which move cyclically from chamber to chamber (in the direction indicated by the arrows) as shown in FIG. 1 (two fire groups being shown). Each fire or fire group is made up of five successive areas A to E, which are as shown in FIG. 1, from downstream to upstream in relation to the direction of flow of the gaseous fluids in the rows of flue walls 6, and in the opposite direction to the cyclical chamber-to-chamber movements:
A) A pre-heating area comprising, with reference to the fire of section 1a and taking account of the direction of rotation of firing indicated by the arrow at the level of the crossover 10 at the end of furnace 1 at the top of FIG. 1:                an exhaust manifold 11 equipped, for each flue wall 6 of the chamber 2 above which this exhaust manifold extends, a system for measuring and regulating the flow rate of the combustion gases and fumes by row of flue walls 6, this system being capable of comprising, in each exhaust pipe 11a which is integral with the exhaust manifold 11 and opening out into the latter on the one hand, and on the other hand engaged in the opening 9 of one respectively of the flue walls 6 of this chamber 2, an adjustable flap pivoted by a flap actuator in order to adjust the flow rate, as well as a flow meter 12, for example of the “Venturi tube” type, and a temperature sensor (thermocouple) 13 for measuring the temperature of the combustion fumes at the exhaust (the flow meter 12 and the thermocouple 13 are only shown in one manifold 11a in FIG. 2 for the sake of clarity); and        a pre-heating measurement ramp 15, situated upstream of the exhaust manifold 11, generally above the same chamber 2, and equipped with temperature sensors (thermocouples) and pressure sensors for measuring the static negative pressure and the temperature prevailing in each of the flue walls 6 of this chamber in order to be able to display and regulate such negative pressure and temperature of the pre-heating area;        
B) A heating area comprising:                several identical heating ramps 16, two or preferably three, as shown in FIG. 1; each equipped with fuel injectors (liquid or gaseous), optionally burners, and temperature sensors (thermocouples), each of the ramps 16 extending above one of the chambers respectively of a corresponding number of adjacent chambers 2, such that the injectors of each heating ramp 16 are engaged in the openings 9 of the flue walls 6 in order to inject the fuel therein;        
C) a blowing or natural cooling area comprising:                a so-called “zero point” ramp 17, extending above the chamber 2 immediately upstream of the one below the furthest upstream heating ramp 16, and equipped with pressure sensors for measuring the static pressure prevailing in each of the flue walls 6 of this chamber 2, in order to be able to adjust this pressure as indicated hereafter, and        a blowing ramp 18, equipped with electric fans provided with a device allowing for the adjustment of the flow of ambient air blown into each of the flue walls 6 of a chamber 2 upstream of the one situated under the zero point ramp 17, so that the flows of ambient air blown into these flue walls 6 can be regulated so as to obtain a desired pressure (slight positive or negative pressure) at the zero point ramp 17;        
D) A forced cooling area, which extends typically over three chambers 2 upstream of the blowing ramp 18, and which comprises, in this example, two parallel cooling ramps 19, each equipped with electric fans and blowing pipes blowing ambient air into the flue walls 6 of the corresponding chamber 2; and
E) A work area, extending upstream of the cooling ramps 19 and allowing for the loading and unloading of the anodes 5, and the maintenance of the chambers 2.
The heating of the furnace 1 is thus ensured by the heating ramps 16, the injectors of which are introduced, via the apertures 9, into the flue walls 6 of the chambers 2 concerned. Upstream of the heating ramps 16 (relative to the direction of fire advance and the direction of circulation of the air and combustion gases and fumes in the rows of flue walls 6), the blowing ramp 18 and the cooling ramp(s) 19 comprise pipes blowing in cooling and combustion air fed by the electric fans, these pipes being connected, via the apertures 9, to the flue walls 6 of the chambers 2 concerned. Downstream of the heating ramps 16, the exhaust manifold 11 is provided for extracting the combustion gases and fumes, denoted as a whole by the term “combustion fumes”, circulating in the rows of flue walls 6.
The heating and baking of the anodes 5 are carried out both by combustion of the (gaseous or liquid) fuel injected, in a controlled fashion, by the heating ramps 16, and, to a substantially equal extent, by the combustion of volatile components of pitch (such as polycyclic aromatic hydrocarbons) diffused by the anodes 5 in the pits 4 of the chambers 2 in preheating and heating areas, these volatile components, a large part of which is combustible, diffused in the pits 4, being capable of flowing in the two adjacent flue walls 6 through degassing gaps arranged in these flue walls in order to ignite in these two flue walls, using the residual combustion air present at this level in the combustion fumes in these flue walls 6.
Thus the circulation of the air and combustion fumes takes place along the rows of flue walls 6, and a negative pressure imposed downstream of the heating area B by the exhaust manifold 11 at the downstream end of the pre-heating area A makes it possible to control the flow of combustion fumes inside the flue walls 6, while a part of the air originating from the cooling areas C and D, via the cooling ramps 19, and the blown air originating from the blowing ramp 18 is preheated in the flue walls 6, cooling the baked anodes 5 in the adjacent pits 4 on its journey, and acts as an oxidant when it reaches the heating area B.
As the baking of the anodes 5 progresses, all of the manifolds and ramps 11 to 19 (with the exception of the two heating ramps 16 that are downstream—before moving—relative to the direction of the fire, since the 3 ramps 16 advance in “caterpillar” fashion, the upstream ramp 16 becoming the downstream manifold of the 3 ramps 16) and the associated measurement and recording equipment and apparatus are advanced cyclically (for example approximately every 24 hours) by one chamber 2, each chamber 2 thus successively providing, downstream of the pre-heating area A, a function of charging the green carbonaceous blocks 5, then, in the pre-heating area A, a function of naturally preheating the blocks 5 by the fuel combustion fumes and pitch vapours that leave the pits 4, entering the flue walls 6, taking account of the negative pressure in the flue walls 6 of the chambers 2 in pre-heating area A, then, in the heating area B or baking area, a function of heating the blocks 5 to approximately 1100° C., and finally, in the cooling areas C and D, a function of cooling the baked blocks 5 by ambient air and, correspondingly, preheating this air constituting the oxidant of the furnace 1, the forced cooling area D being followed, in the direction opposite to the direction of fire advance and circulation of the combustion fumes, by an unloading area E of the cooled carbonaceous blocks 5, then optionally loading of the green carbonaceous blocks in the pits 4.
The method of regulating the BF 1 essentially comprises regulating the temperature and/or pressure of the preheating A, heating B and blowing or natural cooling C areas of the furnace 1, as well, possibly, as steps of optimization of combustion by adjustment of the injection of the fuel by the heating ramps 16, depending on the CO content of the combustion fumes, as measured in the exhaust manifold 11 by at least one CO analyzer-detector provided in at least one of the exhaust pipes 11a. 
In order to ensure the control and monitoring of the BF 1, the instrumentation and control system of the latter can comprise two levels. The first can extend to the set of manifolds and ramps 11 to 19, equipped with sensors and actuators driven by programmable logic controllers, as well as a workshop local network for communication between the logic controllers, as well as for data exchange between the first level and the second, which comprises a central system of computers with their peripheral devices, allowing for communication with the first level, supervision of all of the fires, central regulation of the BF 1, entry of set point rules, management of baking data histories, event management, and storage and production of end-of-baking reports.
Each fire is regulated by row of flue walls 6 from the blowing ramp 18 to the exhaust manifold 11, and, for each row of flue walls 6, the regulation is for example carried out by a regulator of the PID (proportional-integral-derivative) type.
The combustion fumes extracted from the fires by the exhaust manifolds 11 are collected in a fume main 20, for example a cylindrical flue partially shown in FIG. 2, which can be U-shaped in plan view (shown in dotted lines in FIG. 1) or which can surround the furnace, and of which the outlet 22 of the part of the flue closest to the furnace conveys the exhausted and collected combustion fumes to a fume treatment centre (FTC) 23, shown diagrammatically in FIG. 3.
The FTC 23 is an installation for scrubbing the fumes from the BF 1 and performs the following functions:                exhausting the baking fumes emitted by the BF 1, with an almost constant controlled negative pressure,        cooling the fumes,        dry scrubbing of these fumes in order to eliminate the fluorine, dusts and tars contained therein, these pollutant elements being captured in a form that allows for them to be recycled in the aluminium electrolysis cells, and        discharging the scrubbed fumes into the atmosphere.        
The dry scrubbing method is based on the capacity of a powder reagent, generally alumina, to provide physico-chemical neutralization of these pollutants by capturing the fluorine and unburned hydrocarbons by adsorption or catchment. The powder alumina is injected into the stream of combustion fumes originating from the BF 1, then retained in filters at the same time as the dusts, after adsorption and/or catchment of the majority of the pollutants: tars, fluorine gas, sulphur dioxide (SO2). The tar filtration efficiency is higher where heavy elements are concerned (having a high molecular weight, therefore easily condensable), while light tars (not condensed) are contained to a lesser extent. The loaded alumina, recovered by emptying the filters, is then recycled in part by being mixed with fresh alumina and reinjected into the stream of fumes originating from the BF 1, and for the remainder by being sent to the aluminium electrolysis cells, where the combustible elements retained in the loaded alumina are burned, and the fluorine recycled in a directly usable form.
The combustion fume scrubbing functions provided by the FTC 23 are, in order:                cooling, in a cooling tower 24, of the stream of combustion fumes originating from the BF 1,        supplying at least one reactor 25, but preferably several reactors 25 in parallel, with powder alumina (fresh and recycled),        injection of alumina into each reactor 25 with distribution in a stream of combustion fumes passing through the reactor,        catchment-adsorption of the pollutants by the powder alumina distributed in said stream,        filtration of the loaded alumina from the pollutants and dusts of said stream,        recycling of the loaded alumina, by cleaning the filters and removal of the loaded alumina.        
Cooling the fumes consists of reducing their temperature to approximately 100° C. in order to cause condensation of the heaviest and most dangerous unburned hydrocarbons present in these fumes and reduce the temperature of the fumes to a temperature acceptable by the filtration media. This cooling is carried out by total evaporation of water injected in fine droplets in the tower 24 by sprays, as shown diagrammatically in 26, supplied with water by a valve 27 and with air for spraying by a flue (not shown) opening into the water pipe between the valve 27 and the spray 26. This fine spray makes it possible to obtain total evaporation of the injected water and thus to avoid the formation of hydrofluoric acid (HF) or sulphuric acid (H2SO4) by condensation on the internal walls of the tower 24. The flow rate of the valve 27 can, in the state of the art, be controlled by a control loop, which is a feedback loop, in order to tend to keep the temperature of the fumes at outlet of the cooling tower 24 aligned on a temperature set point, while the flow rate of the fumes at the intake of the FTC 23 is measured by a flow meter 28, upstream of the cooling tower 24, on the downstream end of an extension 20′ of the fume main that provides the link with the FTC 23.
There are several types of the water flow regulator, all of which have the aim of injecting a controlled quantity of water, but which, in order to guarantee a good droplet size, use different principles (regulation of the water pressure and constant air pressure, regulation of both pressures at the same time, regulation of the flow rates, etc.).
The circuit supplying the reactors 25 with fresh alumina comprises principally, from a storage and supply silo upstream, a system for grading 32 and metering 33, the outlet of which is linked to a distribution system 34 (the systems for metering 33 and distribution 34 being capable of being grouped together in a single device) providing the distribution of the fresh alumina in an equal manner to the different reactors 25, each of which is thus supplied with fresh alumina.
Catchment of the tar vapours and fluorine gas by the alumina correspond precisely to the conjunction of two different phenomena, which are a surface mechanical catchment of tar droplets and adsorption of tar vapours and fluorine inside the pores of the alumina particles.
This catchment takes place in the reactors 25, in general vertical and having a cylindrical shape or a circular or rectangular cross section. The alumina is injected into each reactor 25 in the most homogenous fashion possible, in order to reduce the average distance between the molecules to be captured and the alumina grains.
After this injection, filtration of the loaded alumina and dusts is provided by the filter cake which forms in the filters 36 (each of which is shown associated with the corresponding reactor 25), generally produced on fabric filter tubes. The cleaning of the filters 36 by blowing air at low pressure, intermittently and in the opposite direction (counter-current to the stream to be filtered), is controlled by the pressure loss of the filters 36 or by a timer. The loaded alumina falls into a fluidized bed maintained in bins of the filters 36, from where a part of this loaded alumina is then reinjected into the reactors 25 while being mixed with fresh alumina, and a part is discharged by an overflow to a means of handling for removal to a silo for recovery of loaded alumina.
The recycling of loaded alumina in the reactors 25 is implemented in order to increase the efficiency of the catchment function.
The scrubbed fumes leaving the reactors 25-filters 36 are discharged into the atmosphere by a stack 43.
Currently, the control and regulation system 44 for the BF 1 and the monitoring and control system 47 for the FTC 23 operate independently of each other.
The fume (exhaust) main 20-20′ is generally a cylindrical steel flue making it possible to extract, by negative pressure, the fumes from the baking of carbonaceous blocks leaving the BF 1 (and extracted by the exhaust manifolds 11 of the active fires), and to convey them to the FTC 23.
The degradation of the fume main 20-20′, as well furthermore as the exhaust pipes 11a and manifolds 11, can result in infiltration of ambient air, therefore colder than the fumes, reducing the average temperature of the fumes and thus promoting condensation and the deposit of unburned volatile matter and acid residues on the internal face of the walls of the fume main 20-20′, in particular.
Such infiltrations of cold air and the possible resultant condensation of a part of the fumes thus promote corrosive action on the metal parts of the fume main 20-20′ and the FTC 23.
Moreover, poor sealing of the fume exhaust flues of the BF 1, and in particular the fume main 20-20′ also constitutes a significant fire risk factor, not only in the fume main 20-20′, but also in the BF 1 and the FTC 23.
The effects of such infiltrations of ambient air and any resulting outbreaks of fire are at the very least, loss of performance and operational disruption, which may even extend to operating losses, when operation of the BF 1 and/or the FTC 23 must be slowed down, as well as the risks of damage and consequent shutdown of the plant in order to carry out any work that may be necessary to repair and make good.
To date, a thorough visual inspection of the different parts of the exhaust ducts of the BF 1 is the only way to detect any unwanted infiltrations of ambient air.
As a general rule, in such installations, preventive inspection and cleaning of the fume main 20-20′ up to the FTC 23 are carried out annually, involving a planned shutdown.
However, these annual inspections do not make it possible to eliminate all fire risks linked to a degradation of the fume main 20-20′, in the interval between two inspections, or linked to defects undetected during these inspections.
The problem to which the invention relates is to remedy the drawbacks mentioned above and make it possible to prevent the infiltration of ambient air into the fume exhaust flues, and in particular the fume main 20-20′, capable of originating deposits of unburned matter by condensation due to the cooling of the fumes, and outbreaks of fire resulting from the fact that the infiltrated air constitutes combustion air that can come into contact with incandescent particles made up of packing coke or pitch volatile matter carried by the fumes, and deposits of pitch volatile matter, generally in the form of heavy tars, which accumulate more or less rapidly over time on the walls of said fume main, leading to loss of performance, operational disruption, even operating losses and damage and in the most extreme cases, shutdowns of the plant made up of a BF 1 connected to a FTC 23 by a fume main 20-20′.